Transcript The History of Atomic Theory

The History of
Atomic Theory
Democritus 460-370 B.C.
• Believed the universe was
made of empty space and
tiny bits of stuff called
atoms that couldn’t be
divided into smaller
pieces.
• The term atom comes
from, atomos, which
means, “cannot be
divided”.
John Dalton-1808
• Believed that
matter was made
of atoms of
different elements.
• Considered the
father of modern
atomic theory.
Thomson's Model
• In 1897, J.J Thomson passed an
electric current through a gas.
• He found that the gas gave off rays
made up of negatively charged
particles.
• Prior to this, atomic theory stated
that atoms are neutral, there must
also be positively charged particles
inside an atom.
Thomson-1897
• Thomson
hypothesized that
an atom was made
up of a positively
charged material
with negatively
charged particles
scattered evenly
throughout.
Thomson's model of an atom
Negatively
charged
material
Positively
charged
material
Rutherford's Model
• In 1911, a British scientist named Ernest
Rutherford discovered that an atom is mostly
empty space.
• He concluded that the positively charged
particles are contained in a small central core
called the nucleus.
• He also concluded that negatively charged
particles were attracted to positively charged
particles found in the nucleus.
• This attraction holds the negatively charged
particles in the atom.
Rutherford-1911
• Discovered the
nucleus.
• Named the
positively charged
particles protons.
• Said that the atom
was mostly empty
space and electrons
traveled around the
nucleus.
Rutherford‘s model of an atom
Bohr's Model
• Rutherford's model of the atom did not explain
the arrangement of electrons.
• In 1913, Niels Bohr proposed that the electrons
in an atom are found in different energy levels.
• Each energy level is at a certain distance from
the nucleus.
• Electrons in different energy levels move around
the nucleus in different orbits.
• Bohr's model explains simple atoms such as
oxygen well, but it does not explain more
complex atoms.
Niels Bohr-1913
• FOUND EVIDENCE
THAT ELECTRONS
WERE ARRANGED IN
ENERGY LEVELS.
Bohr's model of an atom
Gilbert N. Lewis - 1916
• Developed dot
structure diagram of
atoms to explain
how they bond
together.
• Published works on
how atoms combine
or “bond”.
Chadwick & the Nucleus
• When chemists started looking into how much
mass an atom has, they noticed that there is a
mysterious source that they couldn’t figure out
that made the mass bigger than they thought.
• James Chadwick hypothesized and tested to
prove that there is another particle in the
nucleus.
• He found that these particles have no positive or
negative charge.
CHADWICK-1932
• DISCOVERED THE
NEUTRON.
• His ideas allowed for
man-made elements to
be created.
Modern Model
• According to the modern model, the location of
the electrons in an atom cannot be known.
• Therefore, the modern model of the atom does
not show any paths that electrons could be
found in.
• Instead, energy levels are used to predict the
place where an electron is most likely to be
found outside of the nucleus.
• This area is called the electron cloud.
Modern model of an atom
The History of
Atomic Theory
Movie Made with Atoms!
• Regular microscopes cannot give you a visual of
an atom using light to see it.
• Special electromagnetic lenses in special
machines can produce an image of an atom’s
nucleus magnified millions and millions of times.
• With this technology, scientists can manipulate
the location of nuclei to produce images in
sequences that look like a movie.
• You cannot see any electrons. They are 1836
times smaller than a proton.
http://www.youtube.com/watch?v=oSCX78-8-q0